162738282 Refining Crude Oil

8/9/2019 162738282 Refining Crude Oil

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VII-Energy-A-Refining Crude Oil-1

REFINING CRUDE OIL

New Zealand buys crude oil from overseas, as well as drilling for some oil locally. Thisoil is a mixture of many hydrocarbons that has to be refined before it can be used for fuel.All crude oil in New Zealand is refined by The New Zealand Refining Company at theirMarsden Point refinery where it is converted to petrol, diesel, kerosene, aviation fuel,

bitumen, refinery gas (which fuels the refinery) and sulfur.

The refining process depends on the chemical processes of distillation (separating liquids by their different boiling points) and catalysis (which speeds up reaction rates), and usesthe principles of chemical equilibria. Chemical equilibrium exists when the reactants in areaction are producing products, but those products are being recombined again intoreactants. By altering the reaction conditions the amount of either products or reactantscan be increased.

Refining is carried out in three main steps.

Step 1 - Separation The oil is separated into its constituents by distillation, and some of these components(such as the refinery gas) are further separated with chemical reactions and by usingsolvents which dissolve one component of a mixture significantly better than another.

Step 2 - Conversion The various hydrocarbons produced are then chemically altered to make them moresuitable for their intended purpose. For example, naphthas are "reformed" from paraffinsand naphthenes into aromatics. These reactions often use catalysis, and so sulfur isremoved from the hydrocarbons before they are reacted, as it would 'poison' the catalysts

used. The chemical equilibria are also manipulated to ensure a maximum yield of thedesired product.

Step3 - Purification The hydrogen sulfide gas which was extracted from the refinery gas in Step 1 is convertedto sulfur, which is sold in liquid form to fertiliser manufacturers.

The plant at Marsden Point also manufactures its own hydrogen and purifies its owneffluent water. This water purification, along with gas 'scrubbing' to remove undesirablecompounds from the gases to be discharged into the atmosphere, ensures that the refineryhas minimal environmental impact.

INTRODUCTION

Our modern technological society relies very heavily on fossil fuels as an important source ofenergy. Crude oil as it comes from the ground is of little use and must undergo a series ofrefining processes which converts it into a variety of products - petrol for cars, fuel oil forheating, diesel fuels for heavy transport, bitumen for roads.

The NZ Refining Company, situated 40 km from Whangarei at Marsden Point, processesabout 4 500 000 tonnes of oil per year. About 90% of the feedstock comes from overseas,


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and about 10% is the local crude obtained as a byproduct from the production of natural gas

at Kapuni. The sources of oil used are given in Table 1.

In this chapter we will describe the physical and chemical processes by which NewZealand's refinery converts crude oil into the variety of useful products required to meet New

Zealand's needs.

Table 1 - Feedstocks used at the Marsden Point refinery (1992 figures)

Origin Quantity / kT % Wt.

Indigenous feedstock 888 19.8

Foreign crudes 2464 55.2

Synthetic gasoline 419 9.4

Other 700 15.6

Total 4471 100

Uses of refined oil The refinery produces a range of petroleum products. These are listed on the simplified flow

scheme of the refinery in Figure 1, and the relative quantities made are given in Table 2.

Table 2 - Products (1994 figures)

Type Quantity / kT % Wt

Super or Premium petrol 732 15.0

Regular petrol 640 14.6

Jet/DPK 669 15.2

Diesel oil 1482 31.0

Fuel oils 478 10.0

Bitumen 144 3.0

Sulfur 23 1.0

Exports 143 3.0

Refinery fuel and loss 317 7.0

Total 4716 100

Petrol Petrol (motor gasoline) is made of cyclic compounds known as naphthas. It is made in twogrades: Regular (91 octane) and Super or Premium (96 octane), both for spark ignitionengines. These are later blended with other additives by the respective petrol companies.


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Figure 1 - Schematic representation of the Marsden Point Refinery

AGO = Automotive Gas Oil H.C.U. = Hydrocracking unit H.V.U.2 = HB.D.U. = Butane deasphalting unit H.D.S. = K.H.T. =C.D.1 & 2 = Crude distillation units H.D.T. = S.R.U. = SuD.A.O. = Deasphalted oil H.M.U. = Hydrogen manufacturing unit

Asphalt

Waxy dust and DAO

Utilities

H.D.T.

K.H.T.

H.D.S.(as required)

Bitumen

S.R.U.

Platformer

H.M.U.

Crude from tankage

C. D. 1 & 2

H.V.U.2& B.D.U.

Hydrocracker

All Services to Process Units

H 2S from

Process Units

Sour Naptha

Sour Kero

Occasionally Sour G. Oil

Long residue

Sweet G. Oil

Sweet Kero

Sweet Naphtha

Tops

Gas Oil

H2

Platformate

Vacuum G. Oil

H.C.U

Kero Naphtha

Cracker tops


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Jet fuel/Dual purpose kerosene The bulk of the refinery produced kerosene is high quality aviation turbine fuel (Avtur) used by the jet engines of the domestic and international airlines. Some kerosene is used forheating and cooking.

Diesel Oil This is less volatile than gasoline and is used mainly in compression ignition engines, in roadvehicles, agricultural tractors, locomotives, small boats and stationary engines. Some dieseloil (also known as gas oil) is used for domestic heating.

Fuel Oils A number of grades of fuel oil are produced from blending. Lighter grades are used for thelarger, lower speed compression engines (marine types) and heavier grades are for boilersand as power station fuel.

Bitumen This is best known as a covering on roads and airfield runways, but is also used in industry asa waterproofing material.

Sulfur Sulfur is removed from the crude during processing and used in liquid form in themanufacture of fertilisers (see article).

THE REFINING PROCESS

The Marsden Point refinery takes crude oil and refines it by first separating it into its various

constituents, then converting the undesirable elements to desirable ones before purifyingthem to make the final products. These processes are based on some chemical principleswhich are outlined below.

Chemical principles The processes used to refine oil are based on the chemical principles which governdistillation, and require an understanding of chemical equilibria and the effect on reactionrates of catalysts. The theory behind these is outlined below.

Fractional distillation When a mixture of two liquids of different boiling points is heated to its boiling point, the

vapour contains a higher mole fraction of the liquid with the lower boiling point than theoriginal liquid; i.e. the vapour is enriched in the more volatile component. If this vapour isnow condensed, the resultant liquid has also been enriched in the more volatile component.For example, if a 1:1 molar mixture of benzene (b.p. 80.1oC) and toluene (b.p. 110.6oC) isheated it boils at a temperature of 92.2oC and the composition of the vapour (and hence itscondensate) is 71.3 : 28.7 benzene:toluene. If this vapour is condensed and then brought toits boiling point its vapour has a mole ration of 86.4 : 13.6. A third cycle produces acondensate of 94.4 : 5.6 benzene:toluene. This is the principle of batch fractional distillation,and in a distillation column many, many such cycles are performed continuously, allowingalmost complete separation of liquid components. In this process the more volatilecomponents are drawn from the top of the column and the the least volatile ones concentratein the lower part. In practice, the distillation is carried out continuously. A generalised

distillation column is shown in Figures 2 and 3.


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HEAT EXCHANGER -cooling, condensation

SEPARATOR

Uncondensed gases

FURNACE - partialvapourisation

Tray 1

Tray 2

Tray 3

Tray 4

Tray 5

FEED

BOTTOM PRODUCT

HEAT EXCHANGER - partial vapourisation

Reboil

Reflux

Figure 2 - Fractional distillation

LIQUIDS

DOWN-COMER

VAPOURS

PRODUCTSIDE - DRAW

Vapours

Liquids

Figure 3 - Schematic representation of the inside of the distillation column

When two species A and B (A more volatile than B) are fed into the distillation column andheated partial vapourisation occurs. The vapour, richer in A, rises from the feed tray (tray 3)

through the bubble caps, bubbling through the liquid on tray 4, setting up a vapour/liquidequilibrium. Tray 4 is cooler than tray 3, so some of component B will condense, leaving the


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vapour rising from tray 4 richer in A. The same process is repeated on tray 5, and we are leftwith a vapour leaving tray 5 very much richer in component A than was the original feed. Tokeep a temperature differential between the trays, the top vapour leaving tray 5 is condensed by cooling it, and some is fed back to the top of the column, the remainder leaving as the top product.

The liquid portion of the feed entering the column falls from tray 3 to tray 2, which is hotterthan the feed tray 3. On tray 2, because of the increase in temperature, the vapour is richer inA than the liquid, so the composition of the liquid on tray 2 is richer in component B than theoriginal feed. This liquid falls to tray 1, where the process is repeated. To ensure that thetemperature falls from tray to tray going up the column, some of the bottom product (rich inB) is heated and fed back to the bottom tray. The remainder leaves as bottom product rich inB.

Thus, by the use of cooled top product (reflux) and heated bottom product (reboil) thetemperature difference between trays is maintained, and fractional distillation occurs. It canalso be seen that liquid drawn from any of the trays will contain varying concentrations of Aand B, changing from a mixture rich in B at the bottom tray, to a mixture rich in A at the toptray.

In distilling petroleum we are considering not just 2 components, but many components.However, the same principles apply and by feeding heated oil to a fractional distillationcolumn, we can, by withdrawing liquid from various trays, separate the oil into varyingfractions. However, when withdrawing liquid from intermediate trays, some of the light product that bubbles through the liquid on each tray is present. This can be removed by passing steam through the withdrawn fraction in a small distillation column (called a

stripper). The mixture of steam and light material obtained as a top product from the stripperis returned to the main distillation column.

Figure 2 shows only 5 trays with 1 bubble cap per tray. In practice, several tens of trays areused, with of the order of 100 caps per tray.

Chemical equilibrium Many chemical reactions are reversible, i.e. the products react to reform the reactants. Whenthe reactants are mixed products start to form. As the reactant concentrations decrease andthe product concentrations increase, the rate of the forward reaction decreases and the rate ofthe back reaction increases, reforming the reactants. In a closed system where neither

products nor reactants can escape, a state of dynamic chemical equilibrium is eventuallyreached where the rates of the forward and back reactions are equal (thus meaning that thequantities of reactants and products remain the same) and no apparent change is occurring.For the reaction:

aA + bB ! cC + dD

where A and B are reactants, C and D are products and a, b, c, d are the stoichiometriccoefficients (i.e. the numbers in the chemical equation), the concentrations of reactants and products existing at equilibrium are related by the equation:


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where K is called the equilibrium constant. The value of K is extremely important to the

chemist since it shows the extent to which the reactants can be converted into products undera given set of conditions.

Since chemical reactions are employed generally to convert reactants into desirable products,chemists try to convert as much of the reactants as possible into products. Alteration of theconditions of a system is one method whereby the yield of desirable products may beincreased. The effect of changes to the system can be predicted by Le Chatelier's principlewhich states: "If a change is made to a system in equilibrium, the system will adjust itself soas to overcome the effects of the change."

In practice this means that if, for example, reaction products are removed from the system,

the reaction can proceed to completion, i.e. all of A and B can be reacted to produce C and D.Also alterating the pressure of a gaseous reaction mixture, where the total number of moles ofreactants differ from the total number of moles of products, causes the concentrations atequilibrium to change. If the total number of moles of products is less than the total numberof moles of reactants, an increase in pressure will cause the reaction to move to the right.Temperature also affects equilibrium concentrations. The value of the equilibrium constant K changes with temperature. Using Le Chatelier's principle we can see that increase intemperature will cause the reaction to shift to the right if the forward reaction is endothermic(i.e. heat is 'used up' in the reaction).

Reaction Rates and Catalysts Knowledge of the value of the equilibrium constant K at different temperatures is importantto the chemist as it enables prediction of equilibrium concentrations to be made. However,such knowledge does not enable predictions to be made as to the rate at which equilibrium isattained. In industrial processes it is not unusual for equilibrium never to be attained, as thereaction rates are low.

We may consider a reaction taking place via the formation of an activated complex whichdecomposes into the reaction products. Formation of the activated complex requiresrearrangement of the molecular structure of the reactants, which requires additional energy,called the activation energy of the reaction. The further rearrangement of the molecular

structure of the activated complex to form products also has an associated energy change.

Assuming that the decomposition of the activated complex is fast, the rate at which theactivated complex is formed will govern the overall reaction rate. Thus, if a differentactivated complex can be formed, a different reaction rate will result. A catalyst is asubstance that enables a different, easier-to-form activated complex to be formed, and thusincreases the overall reaction rate. Put another way a new faster chemical pathway (ormechanism) from reactants to products is possible. The activated complex can be thought ofas a combination of A, B and catalyst.

][B ][A

][D ][C = K

ba

d c


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Reaction coordinate

Energy A + B

C + D

Energy difference between reactantsand products

Activated complexes

a E 2

a E 1activation energy with catalyst

a E 2

activation energy without catalyst

a E 1

Figure 4 - Reaction pathways with and without a catalyst

The rate of formation of the activated complex with catalyst is faster than the rate offormation of the complex without catalyst. Thus by finding a suitable catalyst, the overallreaction rate can be speeded up. The search for suitable catalysts for various reactions is an

important part of industrial research. (Figure 4 just shows the reaction as one step forsimplicity. In fact both catalysed and uncatalysed reactions probably occur in a series ofsteps, with the activation energy being the difference between the energy of the reactants andthe highest barrier.)

In general all reactions go faster at higher temperature, and the rate of increase withtemperature is greater the higher the activation energy. Thus many reactions which are too

slow at room temperature go at an appreciable rate at elevated temperatures.

Step 1 - Separation The oil is separated by boiling points into six main grades of hydrocarbons: refinery gas(used for refinery fuel), gasoline (naphthas), kerosene, light and heavy gas oils and longresidue. This initial separation is done by distillation. The long residue is further separatedin the butane desaphalting unit, and the refinery gas is separated into hydrogen sulfide in theShell ADIP process.

Distillation The first step in the refining of crude oil, whether in a simple or a complex refinery, is the

separation of the crude oil into fractions (fractionation or distillation). These fractions aremixtures containing hydrocarbon compounds whose boiling points lie within a specifiedrange. A continuous flow of crude oil passes from the storage tanks through a heating coilinside a furnace, where it is heated to a predetermined temperature. The heated oil then

enters the fractionating column (Figures 2 and 3), which is a tall cylindrical towercontaining trays suitably spaced and fitted with vapour inlets and liquid outlets.

Upon entering the column, the liquid/vapour mixture separates - the vapour passing upwardsthrough the column, the liquid portion flowing to the bottom from where it is drawn off as "longresidue". The vapours rise through the tray inlets, become cooler as they rise, and partially

condense to a liquid which collect on each tray. Excess liquid overflows and passes through theliquid outlets onto the next lower tray. The bottom of the column is kept very hot but


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temperatures gradually reduce towards the top so that each tray is a little cooler than the one below it.

Ascending hot vapours and descending cooler liquids mix on each tray and establish atemperature gradient throughout the length of the column. When a fraction reaches a tray wherethe temperature corresponds to its own particular boiling range, it condesnses and changes intoliquid. In this way the different fractions are separated from each other on the trays of thefractionating column and are drawn off for further treatment and blending.

The fractions that rise highest in the column before condensing are called light fractions, andthose that condense on the lowest trays are called heavy fractions. The very lightest fraction isrefinery gas, which is used as a fuel in the refinery furnaces. Next in order of volatility comesgasoline (used for making petrol), kerosene, light and heavy gasoils and finally long residue.

High vacuum distillation unit This works on the principle that lowring the pressure lowers the boiling point of the compoundsconcerned. This is used to separate gasoline into gasoline (which boils under these conditions)

and short residue (a mixture of asphaltic compounds and oil which does not).

The butane deasphalting unit (BDU) The BDU uses a solvent extraction process to separate the gasoline and asphaltic compounds inthe short residue. The short residue from the high vacuum distillation unit is mixed with liquid butane (a 2:1 mixture of n-butane and iso-butane) at a specified temperature and pressure close tothe critical point of butane. Two liquid phases are formed, one rich in butane containing theextracted oil (Deasphalted Oil - DAO), the other of asphaltic compounds with low butane and oilcontent (ASPHALT).

The extraction column operates isothermally (without change in temperature) and has a

continuous butane rich phase with droplets of the asphaltic phase mixed into it. The densitydifference between phases drives the dispersed phase downwards in countercurrent flow to thecontinuous phase which is being forced upwards. This makes an interface level form in the bottom where the two phases are forced past each other. The extractor has perforated trays toenhance the contacting of the two liquid phases, and the butane solvent is recovered from the two phases (DAO and ASPHALT) by flashing and stripping to be recycled for reuse. A diagram of

the BDU is given in Figure 5.

Butane has been selected as the optimum solvent because it provides the highest deasphalted oilyield with good selectabilily at low solvent-to-feed ratios. The butane easily dissolves the lower

boiling hydrocarbons, but its solvent power is limited with respect to the higher boilinghydrocarbons, especially aromatic and asphaltic compounds.


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Figure 5 - The Butane Deasphalting unit

x x x x

EXTRACTION

FLASHINGFLASHING

STRIPPING

STRIPPING

FLASHING

Short residue feed

Butane

Recycling

Exchanger

Steam

Butane

Butane

Compressor

Mixer

Steam

Key

Heating

Pump


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Shell ADIP process The refinery fuel gas contains H2S, and this must be removed from the gas stream before the gasis burnt as fuel to prevent excessive SO2 emissions. This is done by reacting the H 2S with asolution of diisopropanol amine (DIPA, a base) known as ADIP. This is called the Shell ADIP process, and it is a regenerable absorption process, meaning that the ADIP is regenerated for

further use. The overall reaction can be represented as follows:

HS-

N

CH3CHCH2 H

H CH2CHCH3OH

OH

+ +

N

CH3CHCH2

H

CH2CHCH3

OH OH

H2S +

The solution is used to absorb H2S at higher pressures and lower temperatures, during whichtime the equilibrium lies to the right (Le Chatelier's principle). The solution is then regenerated,

releasing H2S, by using higher temperatures and lower pressures. Figure 6 outlines the ADIP process of absorption and regeneration. The DIPA is dissolved in water at a concentration of 4mol L-1, and the H2S concentration of the lean ADIP (the ADIP which is fed into the absorber) istypically 80 ppm wt.

Sour gas

Treated gas

Heatexchanger

LeanADIP Reboil

heater

Steam

Cooling

Water to sour water strippe

H2S to sulphur

recovery

Figure 6 - ADIP process

Step 2 - Conversion Of the oils separated out from the original crude (refinery gas, gasoline, kerosene, light andheavy gasoils and asphalt), only refinery gas can be used as is, and even this is usually ADIP

treated. All the others require some further treatment before they can be made into the final


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As the reaction produces 11 volumes of product from 9 volumes of reactants but the volume iskept constant, the pressure increases, and the rising temperature also caused by the combustioncauses an even greater temperature increase. This increase in pressure provides the motive power. As the pressure increases on ignition, the pressure in the fuel/air mixture in the cylindernot as yet traversed by the flame front rises, with a consequent increase in temperature (PV = n

RT). If the fuel has too low an octane number, this increase in temperature causes the remainingfuel/air mixture to ignite before the flame front passes through. This ignition is explosive, andcauses great impact force to be transmitted by the piston to the bearings. Continual operation ofan engine in which such knocking is occurring causes rapid destruction of bearings. Octanenumbers of hydrocarbons vary with their oxidation stability. Straight chain paraffins have thelowest octane numbers (e.g. n-butane has an octane number of 25) with branched chain paraffins being higher (2 methyl pentane has an octane number of 70) followed by naphthenes (C6H12 =83) and then aromatics (benzene = 100).

The desired reactions (ring forming and aromatising) are endothermic so, to ensure that thereactions take place at an acceptable rate, a temperature of about 500oC is used. At thistemperature all the hydrocarbons considered are gaseous. A pressure of about 25 atmospheres isalso used and the process is carried out using a platinum catalyst (platinum dispersed through1mm spheres of alumina). The process is also sometimes called platforming, because of the useof a platinum catalyst.

Because the reactions are endothermic, 3 furnaces and reactors in series are used. Figure 7 outlines the process.

As the desirable reactions are endothermic, high temperature favours the forward reaction (LeChatalier's principle), which leads to the formation of the desired aromatics. The high

temperature also ensures that the reactions occur at a reasonably fast rate, but in addition favoursthe undesirable cracking reaction (see desulfurisation above), which gives lay-down of carbon onthe catalyst. To suppress coking a high hydrogen pressure is used by recycling some of the lightgases (about 70% vol. of hydrogen) from the separator even though this inhibits the formation ofthe desirable aromatic products to some extent.

As in many industrial processes, this is a compromise. At the previously stated conditions of500 C and 25 atmospheres, conversion of alkylhexanes into alkylbenzenes is fast. Paraffins(long-chain alkanes of more than twenty carbon atoms) are thus driven to be converted toalkylhexanes as the reaction product is removed almost as soon as it is formed. Also,isomerisation reactions that give molecules capable of forming alkyl hexanes are helped, as again

the reaction product is removed.

After a period of several months, deactivation of the catalyst by coke lay-down becomes so pronounced that removal of the coke is necessary. This is achieved by passing a mixture of airand nitrogen through the catalyst, thus burning off the coke. The nitrogen is necessary to controlthe rate of burn off, as if pure air was used the efficiency of the catalyst would be destroyed bythe resulting high temperature. (Steam is not used as in the regeneration of the desulfurisingcatalyst, as water adversely affects the platinum catalyst.) It is interesting to note that during thelifetime of the catalyst each platinum atom leads to the reaction of about 20 000 000 moleculesof gasoline, a truly catalytic act.


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Figure 7 - Catalytic Reforming Unit

A typical reformer feed (bottom product in Figure 2) derived from Middle East crude containsabout 65% wt. paraffins, 20% wt. naphthenes and 15% wt. aromatics. After processing, theanalysis is about 45% wt. paraffins1, virtually no naphthenes and 55% wt. aromatics. The octanenumber of the reformer feed is about 50, while the octane number of the product is about 100.The product from the separator contains light gases from partial cracking, and these are removed by fractional distillation.

R CH3

R R

R R

Paraffins Naphthenes Aromatics

(R consists of branched or unbranched alkane chains)

Bitumen manufacture Bitumen used to be made by a process that combined distillation and oxidation. Currently it ismade simply by mixing some of the short residue from the high vacuum unit with asphalt from

the BDU.

Hydrocracking The hydrocracking process is used to convert waxy distillate and deasphalted oil (DAO) intokerosine and gasoil by breaking down some of their constiuents. The process is carried out intwo stages, the first to reduce the amount of nitrogen, sulfur and oxygen impurities that may

1Long-chain alkanes of more than twenty carbon atoms.

reach the second stage catalyst, and the second to continue the process of cracking,hydrogenating and isomerising the compounds in the oil. The reactions occuring aredenitrogenation, desulfurisation, deoxygenation, hydrogenation, hydrocracking and

FEED

Heatexchanger

Furnace

Reactor

Compressor

Air cooler

Recycle hydrogen

Separator

PRODUCT


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isomerisation, all of which are exothermic and all of which, except for isomerisation, consumehydrogen. The heat released is absorbed by injecting cold hydrogen quench gas between thecatalyst beds. Without the quench the heat released would generate high temperatures and rapidreactions leading to greater heat release and an eventual runaway. All of the reactions, except fordenitrogenation, desulfurisation and deoxygenation which only occur in the first stage, happen in

both stages.

DenitrogenationCarbon-nitrogen bonds are ruptured with the formation of ammonia.

N

+ 5 H2 H3C CH2 CH2 CH2 CH3 + NH3

Pyridine n-pentane ammonia

DesulfurisationCarbon-sulfur bonds are ruptured with the formation of hydrogen sulfide.

H3C CH2 CH2 CH2 SH + H2 CH3 CH2 CH2 CH3 + H2S

Butyl mercaptan n-butane hydrogen sulfide

DeoxygenationCarbon-oxygen bonds are ruptured with the formation of water.

OH

+ H2 + H2O

phenol benzene

HydrogenationThe saturation of carbon-carbon double bonds of the olefins or aromatics.

H3C CH2 C C

HH

H

+ H2 H3C CH2 CH2 CH3

butylene n-butane


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+ 2H2

naphthalene tetralin

decalin

CH2CH2CH2CH3H2

H23

tetralin butyl benzene

HydrocrackingThe splitting or breaking of straight or cyclic hydrocarbons and hydrogenation of theruptured bonds.

CH2CH2CH2CH3

H2 CH3CH2CH2CH3+ +

butyl benzene benzene n-butane

IsomerisationThe change of one compound into another by rearrangement of its atoms.

CH3CH2CH2CH2 CH3CHCH3

CH3

n-butane iso-butane

Condensation

All the previous reactions yield more or less useful products. A less desirable but importantreaction is the condensation of aromatics to produce large unsaturated polyaromatics whichdeposit on the catalyst to form coke. Polyaromatics are much more susceptible to thisreaction than monoaromatics. Hence rapid catalyst deactivation due to coke laydown willoccur if the feedstock contains a high amount of polyaromatic asphaltenes.

2 H2+ 2

anthracene hexacene


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An outline of the hydrocracking unit is given in Figure 8.

Step 3 - Purification The crude oil has now been separated into refinery gas, hydrogen sulfide, naphtha, kerosene,gas oil, asphalt and bitumen. Two more processes have to be carried out, on the naphtha and

the hydrogen sulfide respectively, before the hydrocarbons are ready for blending intosaleable products.

Sulfur recovery Crude oil as received for refining contains sulfur in levels up to a few tenths of a percentweight. It is removed from oil products mainly by the desulfurisation process describedabove which results in the formation of hydrogen sulfide, and further H 2S is separated out ofthe refinery gas. This H2S is converted to sulfur in a two step process. Firstly, the "Claus" process of partial combustion of H2S is used to form SO2 and this SO 2 is then reacted withthe remaining H2S to produce sulfur. This sulfur recovery process takes place in one thermaland two catalytic stages and recovers 95% of the sulfur. The final 1 or 2 % volume of H2S inthe "tail gas" from the last catalylic reactor is burnt in a separate incinerator so that theeffluent gas finally discharged to the atmosphere has an environmentally acceptable H2Scontent of less than 5 ppm by volume.

The overall reaction occuring is as follows:

H2S + ½O2 ! S + H2O (l)This overall reaction (1) is the sum of two exothermic reactions, the oxidation of H2S to SO2 (2) and the subsequent reaction between H2S and SO2 to form sulfur and water (3):

H2S + 1½O2 ! SO 2 + H 2O (2)

2H2S + SO2 ! 3S + 2H 2O (3)

In the first stage of the process (the 'thermal stage'), enough air is supplied to convert onethird of the H2S in the acid feed gases to SO2 and H 2O according to equation (2). In additionto this, any hydrocarbons and NH3 in the feed gases are completely combusted:

C3H8 + 5O 2 ! 3CO 2 + 4H2O (4)

NH3 + ¾O 2 ! ½N 2 + 1½H 2O (5)

In the second and the third stage of the "Claus" process more H2S is converted to sulfuraccording to equation (3). To shift the equilibrium of this reaction as far as possible to theright side lower reaction temperatures are applied to these stages. To assure sufficient high

reaction rates, the reactions take place in the presence of a catalyst.

Finally, the SRU tail gas (which contains less than 5% sulfur) is oxidised in a catalyticincinerator at a temperature of approximately 400oC. At this temperature, achieved by burning fuel gas in addition to the process gases, the H2S and sulfur vapour/mist are practically completely oxidised in the presence of a catalyst according to the reactions:

H2S + 1½O2 ! H 2O + SO2

S + O2 ! SO 2

The sulfur is produced in liquid form and heated handling/loading facilities provide sulfur

storage before loading into road tankers for delivery to fertiliser works. Figure 9 gives an

outline of the SRU process of the refinery.


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Waxydistillate from HVU-2

DAOfromBDU

FIRST STAGE REACTORS

SECOND STAGE

REACTORS

H2

H2

Residue recycle (to extinction)

H2 quench

H2 quench

Cooling

Hydrogenrecycle gas

Compressor

2 stageflashing

Reboilheater

Cooling

Distillation

Figure 8 - The two-stage hydrocracking process


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Figure 9 - The sulfur recovery unit

H2S

fuel gas

Air Air

fuel gas

1st stage(THERMAL)

2nd stage(CATALYTIC)

3rd stage(CATALYTIC)

Condensor

Air

fuel gas

Condensor

fu

Sulphur pit Pump

Sulphur loading tank


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ANCILLARY PROCESSES

Hydrogen manufacture The large consumption of hydrogen, particularly in the hydrocracker, has meant that the Marsden

Point refinery has its own hydrogen manufacturing unit (Figure 10). The hydrogen is produced

by converting hydrocarbons and steam into hydrogen, and produces CO and CO2 as byproducts.The hydrocarbons (preferably light hydrocarbons and butane) are desulfurised and then undergothe steam reforming reaction over a nickel catalyst. The reactions which occur during reformingare complex but can be simplified to the following equations:

CnHm + nH 2O ! nCO + (( 2n + m )/2)H 2

CO + H2O ! CO 2 + H2

The second reaction is commonly known as the water gas shift reaction.

The process of reforming can be split into three phases of preheating, reaction and superheating.

The overall reaction is strongly endothermic and the design of the HMU reformer is a carefuloptimisation between catalyst volume, furnace heat transfer surface and pressure drop.

In the preheating zone the steam/gas mixture is heated to the reaction temperature. It is at theend of this zone that the highest temperatures are encountered. The reforming reaction then

starts at a temperature of about 700"C and, being endothermic, cools the process. The final phase of the process, superheating and equilibrium adjustment, takes place in the region wherethe tube wall temperature rises again.

The CO2 in the hydrogen produced by reforming is removed by absorption (see purification below), but trace quantities of both CO and CO2 do remain. These are converted to methane

(CH4) by passing the hydrogen stream through a methanator. The reactions are highlyexothermic and take place as follows:

CO + 3H2 ! CH 4 + H 2O

CO2 + 4H 2 ! CH 4 + 2H 2O

Finally, all produced hydrogen is cooled and sent to the Hydrocracker.

In contrast to the desulfurisation process described earlier, the HMU is an example of anon-reversible absorption process applied to protect a valuable catalyst. In this case,desulfurisation is by absorption of H2S in zinc oxide, according to the equation:

ZnO + H2S ! ZnS + H 2O

The reaction is non-reversible and consequently the saturated absorbent must be discharged andreplaced.


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High temperatureshift converter Low temp.

shiftconverter CO 2

removal sulfinol

absorption

process

Cooling Cooling Heating Cooling

Methanator

Cooling

Mixer

Reformer furnaceVapourised feed Desulphurisation

A B

Steam

Figure 10 - The hydrogen manufacturing unit

Purification The hydrogen produced must be purified (to remove the carbon oxides) before it can be usd inthe hydrocracker. A regenerable absorption process is applied by circulating a "Sulfinol"solution. This is a mixed solvent, consisting of 50% DIPA, 25% water and 25%tetraydrothiophenedi-oxide (sulfolane).

SOO

sulfolane

CO2 is removed by absorption in the sulfinol at low temperatures and high pressure.Subsequently, the "fat" sulfinol solution is regenerated by removal of CO2 at high temperatureand low pressure. The CO2 is further purified by chilling to give food grade product. Thechemistry of the process is as follows:

CO2 + 2

N

CH3CHCH2

H

CH2CHCH3

OH OH N

CH3CHCH2 HOH

CH3CHCH2 H

OH

++

N

CH3CHCH2

C

CH2CHCH3

OH OH

O

O-

DIPA


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CO2 + H2O +

N

CH3CHCH2

H

CH2CHCH3

OH OH

HCO3-

+

N

CH3CHCH2 H

H CH2CHCH3OH

OH

+

N

CH3CHCH2

C

CH2CHCH3

OH OH

O

O-

+ N

CH3CHCH2 H

H CH2CHCH3OH

OH

+

CH2

CCH3

H O

C

O

N CH2 CH

OH

CH3 H2O

N

CH3CHCH2

H

CH2CHCH3

OH OH

+ +

oxazolidone

The water treatment plant A major ancillary facility of the expanded refinery is the effluent water treatment plant.Extensive facilities, costing $50 million (1985), ensure the continued protection of the marineenvironment of Whangarei Harbour. The effluent water discharge is continuously monitoredfor oil content and other contaminants, according to the comprehensive requirements of the

discharge permit issued by the Northland Regional Council. Figure 11 is a simplified blockflowscheme of the effluent water treating facilities.

There are two types of water to be dealt with:

# Water which has been, or is likely to have been, in contact with oil. This water must pass through gravity separators to remove the oil and may require other treatment.Such water comes from the refining processes or ballast discharged by coastaltankers.

# Water which has only accidentally been in contact with oil. This water passesthrough oil traps before being routed to the holding basin and final buffer basin. Suchwater comes from the boiler plant and collected rainwater.

The treatment of effluent water is as follows. Process water is deodorised in sour-waterstrippers where the gas (H2S and NH3) is stripped off. The stripped water has oil removed inthe gravity separators and then, together with some rainwater, is homogenised in a buffertank. From this tank, the effluent water is piped to a flocculation/flotation unit where air and polyelectrolytes are injected in small concentrations to make the suspended oil and solidsseparate from the water. The latter are skimmed off and piped to a separate sludgehandling/disposal unit. The remaining watery effluent from the flotation unit is passed toadjoining biotreater where the last of the dissolved organic impurities are removed by theaction of micro-organisms in the presence of oxygen (biodegradation). On a continual basis,sludge containg micro-organisms is removed to the sludge handling/disposal unit, while thetreated effluent water is held first in the retention basin and finally discharged into Whangarei

Harbour.


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Process units Drainage

Continuous

ex utility plant

Continuous Intermittent

BALLAST WATER

SEWAGE WATER ACCIDENTALLY

CONTAMINATED BY OIL

Sour water stripper

Gravity oil separator

Oil trap

Bio treater

Processing areas

Metering/sampling point

WATER EXPECTED TO BE

CONTAMINATED BY OIL

OIL FREE WATER

Receiving tanks


Comminutors


Homogenisation tank

Flocculationflotation unit

Aeration basin

Holding basin

Buffer basin

DISCHARGE

OF TREATED EFFLUENT

TO MARINE OUTFALL

Figure 11 - The water treatment plant

The discharge is through a continuous flow meter and sampling system and terminates attwin diffusers located in deep water where tidal currents ensure the rapid and extensivedilution of the treated effluent water.

A comprehensive system of waste management has also been established at the refinery.This is based on internationally accepted concepts of waste minimisaiton, such as reduction,reuse and recycling. The disposal of any remaining residual waste follows strictly controlled procedures.

ENVIRONMENTAL IMPLICATIONS

The refinery operates under an air discharge permit, monitored by the Northland RegionalCouncil (NRC), since the enactment of the Resource Management Act (RMA). The current


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permit requires monthly reports to be prepared for the NRC, relating to sulfur dioxide (SO2)emissions, ground level measurement of SO2, smoke from chimneys, from flaring and fromfire fighting training.

The permit also required that the quantity of SO2 emitted should be significantly reduced in

three steps between 1992 and 1996. Capital costs to achieve these reductions were in theregion of $30 million. In addition, operating costs of several million per annum wereincurred. The SO2 limit is now amongst the lowest of any similar refinery in the world.

In addition, the water treatment plant described above ensures that minimal quantites of oiland other effluent are discharged with the water into the Whangarei Harbour.

Article written by Heather Wansbrough, combining the articles from volumes one and two ofedition one and information brochures supplied by Tony Mullinger (The New ZealandRefining Company Ltd). Edited by John Packer and John Robertson.

Documents

162738282 Refining Crude Oil